It is crucial to keep the lampshade in proportion to the lamp base, because a lampshade that is too large will become top-heavy and a lampshade that is too small will always look unbalanced.
A great tip for figuring out the best shade size for your lamp is to measure the height from the base to the top, excluding the lamp holder. Use this height measurement as a guide for your lampshade's diameter.
Remember that this design is just a guide and not a one-size-fits-all, it all depends on your personal style and taste.
Lampshade sizing example: If your lampshade is 30 cm tall, the ideal diameter would be 30 cm. Or, if you prefer a smaller size, it would be 25 cm in diameter, and a larger size would be 35 cm in diameter.
The important thing is to find the sweet spot between form and functionality.